In order to drive the main lift and propulsion rotor of a rotorcraft, such as a helicopter, via a main power gearbox, a first known solution relies on using a turbine engine known as a “free turbine engine”.
A free turbine engine includes a gas generator comprising in succession at least one compressor, a combustion chamber, and at least one expansion turbine, the compressor being mechanically linked to the expansion turbine by a main shaft.
It is possible to provide the compressor with a plurality of compression stages that may be axial and/or centrifugal, and the expansion turbine may present a plurality of expansion stages.
Following on from the gas generator, there is arranged a working free turbine that is secured to the outlet shaft of the turbine engine. The main shaft of the gas generator and the outlet shaft are then distinct. Consequently, a free turbine engine is sometimes referred to as a “two-shaft” engine.
In operation, the turbine engine sucks in ambient air. The air is then compressed prior to being directed into the combustion chamber.
A fuel is injected under pressure into the combustion chamber and then burnt together with the compressed air.
The gas that results from the combustion is then taken to the expansion turbine where it is expanded.
This gas thus drives the expansion turbine in rotation about its axis of rotation.
It should be observed that the expansion turbine drives the compressor of the gas generator in rotation by means of the main shaft.
The gas coming from the expansion turbine of the gas generator then sets the power free turbine into rotation, which in turn drives the outlet shaft of the turbine engine, and consequently drives the main power gearbox.
In order to start a free turbine engine, it is possible to use a starter that sets the main shaft of the engine into rotation.
Since the main shaft is not secured to the outlet shaft in a “two-shaft” engine, the starter can be optimized and there is no necessity for the starter to be overdimensioned.
Furthermore, and with reference to document WO 2007/086906, it is appropriate to arrange a freewheel on the outlet shaft in order to enable a rotorcraft having a turbine engine to perform autorotation.
Thus, that document describes a turbine engine suitable for driving a special transmission gearbox having a main gearbox and a variable-speed gearbox, the variable-speed gearbox enabling the turbine engine to drive a rotor at two different speeds.
A freewheel is then arranged on the outlet shaft from the turbine engine, between the body of the turbine engine and the variable speed gearbox.
Thus, in the event of the turbine engine breaking down, the freewheel guarantees that if the free turbine of the turbine engine is being prevented from rotating that will not also cause the main rotor from being prevented from rotating.
It should be observed that document WO 2007/086906 implements conventional freewheels.
A conventional freewheel is usually provided with a driving portion and a drivable portion, at least one ball being arranged between a sloping ramp of the driving portion and a circular surface of the drivable portion. Thus, when the drivable portion is rotating faster than the driving portion, said ball is at the bottom of the sloping ramp, thereby disconnecting the drivable and driving portions.
Conversely, when the driving portion is rotating faster than the drivable portion, the ball moves up the sloping ramp and thus becomes wedged between the driving portion and the drivable portion. Consequently, the driving portion sets the drivable portion into rotation via said ball. The drivable and driving portions are then constrained to rotate together.
In addition to the conventional freewheels implemented in document WO 2007/086906, freewheels are also known that are suitable for being disconnected and that are entirely different from a conventional simple freewheel. A freewheel that is suitable for being disconnected operates in two distinct connection modes:                a disconnected mode, in which the driving portion of the freewheel is unable under any circumstances to set the drivable portion thereof into motion; and        a freewheel mode, in which the driving portion of the freewheel sets the drivable portion thereof into rotation as soon as the first speed of rotation of the driving portion becomes greater than a second speed of rotation of the drivable portion, whereas in contrast said driving portion of the freewheel is not suitable for setting said drivable portion into rotation when the first speed of rotation is less than the second speed of rotation.        
Reference may be made to the literature in order to find embodiments of a freewheel that is suitable for being disconnected.
By way of example, in a first embodiment described in document FR 2 670 553, the balls of a conventional freewheel are replaced by rollers arranged in a cage that is movable along the longitudinal axis of the freewheel. Thus, in disconnected mode, in a first position, the rollers are not between the driving and drivable portions and can under no circumstances transmit motion from the driving portion to the drivable portion. In contrast, in freewheel mode, the rollers are in a second position where they are arranged between the driving and drivable portions so as to be capable of transmitting motion from the driving portion to the drivable portion, where appropriate.
Compared with conventional freewheels, the term “declutchable” freewheel is used for convenience below in order to designate such freewheels that are suitable for being disconnected.
It should be observed that a declutchable freewheel is sometimes referred to by the person skilled in the art as a “disconnectable” freewheel (roue libre “décrabotable” in French language). Indeed, the succession of ramps on its driving portion gives rise to a succession of dog, i.e. dog-clutch-like faces between each adjacent pair of ramps.
In addition to free turbine engines, it is possible to use a turbine engine that may be referred to as a “linked turbine” engine.
A linked turbine engine is provided with a gas generator of the type described above.
In operation, the engine sucks in ambient air. This air is then compressed prior to being directed into the combustion chamber.
Fuel is injected under pressure into the combustion chamber where it is burnt with the compressed air.
The gas that results from the combustion is then taken to the expansion turbine in order to be expanded.
This gas then drives said expansion turbine in rotation about is axis of rotation.
The expansion turbine drives the compressor of the gas generator in rotation by means of the main shaft.
In addition, the main shaft serves not only to link the compressor to the expansion turbine. In a linked-turbine engine, the main shaft also constitutes the outlet shaft from the engine, and is suitable for setting a main gearbox into motion, e.g. in a helicopter.
Thus, linked-turbine engines are sometimes referred to as “single-shaft” turbine engines.
In order to start a linked-turbine engine, an external starter is used that sets the compressor into rotation. However, unlike a free turbine engine, the main shaft and the outlet shaft constitute a single shaft, so the starter must also drive the outlet shaft and thus the main rotor in rotation as well.
Consequently, the starter needs to be overdimensioned in order to perform its function, thereby giving rise to unreasonable cost, size, or indeed fuel consumption, and also to excessive weight.
This drawback explains in particular why linked-turbine engines are rarely used in rotorcraft, where nominal weight targets in particular are recurrent for this type of aircraft.
The same applies to piston engines whether reciprocating or rotary, since they present the same drawbacks.
In order to overcome that difficulty, it is common practice to install a clutch, of the centrifugal friction type, on the outlet shaft. A driving portion of the clutch is then secured to the outlet shaft while a driven portion of the clutch is secured to a shaft for driving the gearbox.
On starting, the clutch is declutched. Thus, the outlet shaft is no longer connected to the main rotor, thus making it possible to optimize the performance of the starter.
In contrast, once the engine has started, whether it is the turbine engine or a piston engine, the clutch is clutched so that the outlet shaft can drive the main rotor via the main gearbox.
Although effective, that device presents drawbacks. After starting, the clutch is subjected to high levels of torque and runs the risk of its driving portion slipping relative to its driven portion. This gives rise to non-negligible levels of wear in the device.
Similarly, in flight, the risk of slipping remains, given the presence of large fluctuations in torque.
Consequently, the clutches are greatly overdimensioned in order to avoid any risk of slip, and they require frequent and expensive maintenance operations.